Phylum: Mollusca Mya arenaria Class: Bivalvia, Heterodonta, Euheterodonta Order: Imparidentia, Myida Soft-shelled clam Family: Myoidea, Myidae Taxonomy: Mya arenaria is this species Body: Body is egg-shaped in outline (Fig. 1; original name and is almost exclusively used Ricketts and Calvin 1952) (see Fig. 305, Ko- currently. However, the taxonomic history of zloff 1993). this species includes many synonyms, over- Color: lapping descriptions, and/or subspecies (e.g. Interior: A crystalline style (consisting Mya hemphilli, Mya arenomya arenaria, of a gelatinous cortex and liquid core, Lawry Winckworth 1930; Bernard 1979). The sub- 1987) resides in a sac lined with cilia. The genera of Mya (Mya mya, Mya arenomya) cilia allow the style to rotate and press against were based on the presence or absence of a a gastric shield within the stomach, aiding in subumbonal groove on the left valve and the digestion (Lawry 1987). In M. arenaria, the morphology of the pallial sinus and pallial crystalline style can be regenerated after 74 line (see Bernard 1979). days (Haderlie and Abbott 1980) and may contribute to the clam’s ability to live without Description oxygen for extended periods of time (Ricketts Size: Individuals range in size from 2–150 and Calvin 1952). The ligament is white, mm (Jacobson et al. 1975; Haderlie and Ab- strong, and entirely internal (Kozloff 1993). bott 1980; Kozloff 1993; Maximovich and Two types of gland cells (bacillary and goblet) Guerassimova 2003) and are, on average, comprise the pedal aperture gland or glandu- 50–100 mm (Fig. 1). Mean weight and lar cushion located within the pedal gape. It is length were 74 grams and 8 cm situated adjacent to each of the two mantle (respectively) in Wexford, Ireland (Cross et margins and aids in the formation of al. 2012). Individual weight varies seasonal- pseudofeces from burrow sediments; the ly and is greatest just before spawning and structure of these glands may be of phyloge- the smallest just after (range, 100–200 mg netic relevance (Norenburg and Ferraris ash-free dry weight, Wadden Sea, Zwarts 1992). 1991). Exterior: Color: White with gray or dark, yellowish Byssus: brown periostracum on shell edges, creating Gills: a rough outermost layer. Siphons are dark Shell: Shell is soft, thin, fragile (hence “soft (Haderlie and Abbott 1980; Kozloff 1993; shell clam”, Kozloff 1993; Coan and Valentich see Fig. 3, Zhang et al. 2012). -Scott 2007), and composed completely of General Morphology: Bivalve mollusks are aragonite (MacDonald and Thomas 1980). bilaterally symmetrical with two lateral The valves have an oval and rounded anterior valves or shells that are hinged dorsally and and a pointed posterior (Kozloff 1993) and surround a mantle, head, foot and viscera gape at each end (Haderlie and Abbott 1980). (see Plate 393B, Coan and Valentich-Scott External shell sculpture is with concentric 2007). Myoid bivalves are burrowers and rings (Fig. 1). borers, with long siphons and hinges with Interior: Deep pallial sinus and spoon- few teeth (Coan and Valentich-Scott 2007). A publication of the University of Oregon Libraries and the Oregon Institute of Marine Biology Individual species: https://oimb.uoregon.edu/oregon-estuarine-invertebrates and full 3rd edition: http://hdl.handle.net/1794/18839 Email corrections to: [email protected] Hiebert, T.C. 2015. Mya arenaria. In: Oregon Estuarine Invertebrates: Rudys' Illustrated Guide to Common Species, 3rd ed. T.C. Hiebert, B.A. Butler and A.L. Shanks (eds.). University of Oregon Libraries and Oregon Institute of Marine Biology, Charleston, OR. shaped chondrophore, or triangular projec- stone (Sphenia) or those utilizing the burrows tion, is present on the left valve only of other species (Cryptomya, Paramya) (Haderlie and Abbott 1980; Kozloff 1993). (Zhang et al. 2012). Characters of the Myidae Left and right adductor muscle scars are the include a shell that is not cemented to the same size but very different in shape (Fig. substratum, valves that are (relatively) mor- 2). phologically similar, a dorsal margin without Exterior: Left and right valves are of ears, a hinge with an internal ligament in a similar morphology, which is long and egg- distinct resilifer or chondrophore that is spoon shaped, with shells convex, thin and brittle shaped and present on the left valve (Coan (Fig. 4). Low concentric growth striae on and Valentich-Scott 2007). Cryptomya anterior and posterior ends are different: species are characterized by hinge without anterior are more blunt and posterior are tooth-like process anteriorly on the right valve. pointed, but both ends gaping (Packard Mya, on the other hand, have thin shells, ga- 1918). Beaks small, bent posteriorly, and ping anteriorly and posteriorly and commargi- slightly anterior of center (Fig. 2). nal growth lines (Zhang et al. 2012). Hinge: Valve areas dissimilar and There are only three local myid species with spoon-shaped chondrophore in left including Platyodon cancellatus, Mya arenaria valve. Right valve is with tooth in opposition and Cryptomya californica (“the false Mya” to chondrophore (Fig. 3). No hinge plate see description in this guide). Platydon can- teeth (cardinal or lateral). cellatus can be distinguished from the latter Eyes: two species because its shells are heavy and Foot: with wavy commarginal sculpture and a round Siphons: Long, large siphons are fused, anterior. It has a truncate, gaping posterior non-retractable (Coan and Valentich-Scott end covered with periostracum. It also bores 2007; Tan and Beal 2015), and dark in color into rock and hard clay while M. arenaria and (Haderlie and Abbott 1980). C. californica burrow into sand or mud. The Burrow: Unlike the other local member of shells of the two latter species are relatively the Myidae, Cryptomya californica (see de- thin. In M. arenaria, the pallial sinus is deep scription in this guide), M. arenaria has long and individuals reach sizes of 150 mm, while siphons and can be found in relatively deep in C. californica the pallial sinus is shallow, burrows up to 40 cm (Haderlie and Abbott inconspicuous and individuals tend to be 1980; Kozloff 1993; Coan and Valentich- smaller (30 mm) (Coan and Valentich-Scott Scott 2007; González et al. 2015). 2007). Mya arenaria is found as deep as 40 cm and is not necessarily near Callianassa Possible Misidentifications californiensis burrows, where one might find There are five bivalve subclasses Cryptomya californica. The siphons are M. based on morphology and fossil evidence arenaria are also longer than those of C. cali- and one of those is the diverse Heterodonta. fornica (see C. californica, Figs. 1, 6 in this Recent molecular evidence (18S and 28S guide). Additionally, Sphenia luticola is a myid rRNA) suggests that the heterodont order species that may occur in our area, but is Myoida is non monophyletic (Taylor et al. found offshore in rocks and within kelp hold- 2007). The family Myidae includes 25–40 fasts (Coan and Valentich-Scott 2007). Juve- species worldwide, which can be divided in- nile Mya are not easily distinguished from to groups such as those that are burrowing Sphenia species, but Mya can be recognized (Mya), those that are attached to shells or by a large continuous pallial sinus (Coan A publication of the University of Oregon Libraries and the Oregon Institute of Marine Biology Individual species: https://oimb.uoregon.edu/oregon-estuarine-invertebrates and full 3rd edition: http://hdl.handle.net/1794/18839 Email corrections to: [email protected] 1999). and Abbott 1980; Ricketts and Calvin 1952). Mya arenaria may be confused with Common on the Atlantic Coast and Europe in other local common clams, e.g. Saxidomus, areas of low salinity (e.g. Baltic Sea, Kozloff Tresus, Tellina or Macoma species. These 1993). It has crowded out the native Macoma genera do not have an internal ligament or a spp. on the Pacific coast in some areas (Keep chondrophore. Small Tresus can otherwise and Longstreth 1935). In the Cold Temperate be mistaken for M. arenaria. Small Tellinid Northwest Atlantic biogeographic province, six clams have an external ligament without a genetic clusters of M. arenaria were observed nymph, and lateral hinge teeth, which M. spanning seven distinct ecoregions. Those to arenaria lack. Macoma species (see de- the north were defined by geographic barriers scriptions in this guide) are very like Tellina, and selection processes and those to the but their shells are always a bit flexed, they south were likely the result of and increased have no lateral teeth, and no internal colora- with geographic distance only (St-Onge et al. tion. Furthermore, where M. arenaria is 2013). abundant is in upper reaches of estuaries Local Distribution: Local distribution in Coos where salinity is reduced, species in the and Yaquina Bay as well as the Suislaw, genera Saxidomus and Tresus are not usua- Umpqua, Tillamook, Alsea and Columbia es- lly found. tuaries. Habitat: Mud and sand of bays with sand, Ecological Information mud, gravel mix (Kozloff 1993; Coan and Range: Type locality is Europe (Zhang et al. Valentich-Scott 2007), often in upper reaches 2012). Current eastern Pacific distribution where salinity is reduced, but requires com- from Alaska to San Diego, California plete protection, as it cannot burrow or main- (Haderlie and Abbott 1980). Current popula- tain itself in a shifting substratum (Ricketts tions introduced from the Atlantic coast with and Calvin 1971). Very tolerant of extreme oyster spat in 1874 in San Francisco (Coan conditions (e.g., anaerobic or foul mud, brack- and Valentich-Scott 2007), although it ish water, temperatures below freezing, Rick- appears in the fossil record (Ricketts and etts and Calvin 1971; Haderlie and Abbott Calvin 1971) in California and Vancouver 1980). Can live without oxygen for eight days (Packard 1918). However, M. arenaria is (Ricketts and Calvin 1952) and it is thought not represented in local Native American that the shell serves as an alkaline reserve to mounds (Kozloff 1993).